A sudden drop in fuel economy does not automatically mean the engine is failing. Mileage is the output of a system: speed, temperature, tire pressure, traffic, cargo, idling and mechanical condition all move the number. Diagnose the change before you buy a fix.
Did your driving speed change?
Higher highway speed costs fuel because aerodynamic drag rises quickly. U.S. Department of Energy material notes that fuel economy generally falls as speed climbs; one ORNL analysis cited by DOE found fuel economy at 80 mph roughly 27% lower than at 60 mph across the vehicles tested.
Weather and trip length can make the same car look worse
Cold weather
Engines and fluids take longer to reach efficient operating temperatures, tire pressure can fall, and heating/defrost loads increase. DOE testing has shown meaningful city-mpg penalties in cold conditions.
Short trips
If most drives end before the vehicle fully warms up, the cold-start penalty becomes a larger share of every mile.
Look above the roof — then look at the tires
Roof boxes and racks increase aerodynamic drag, particularly at highway speed. Underinflated tires increase rolling resistance. Check pressure against the vehicle manufacturer’s specification, not the maximum printed on the tire sidewall.
If the mileage drop began after adding roof cargo, switching tires or a major temperature change, test those variables before assuming a deeper fault.
Traffic and idling can wreck the average without changing the route
Stop-and-go traffic adds acceleration losses and time spent burning fuel without covering distance. A commute can therefore produce worse mpg even if the odometer shows the same journey.
Then consider maintenance
A serious mechanical problem can hurt fuel economy. Warning lights, rough running, dragging brakes or a persistent unexplained decline deserve proper diagnosis. Routine maintenance should follow the vehicle schedule; replacing parts at random is not a fuel strategy.
Treat MPG like a diagnostic signal
Compare similar tanks, routes and weather. One bad week is noise; a repeatable change is information. The fastest way to improve fuel economy is often identifying which variable changed first.